1. Field of the Invention
The present invention relates to a TDD type transmitter-receiver for use in both home and public cordless telephone systems
2. Description of the Prior Art
In a digital cordless telephone system such as a telepoint system, there is generally employed a TDD or TDMA/TDD method which executes ping-pong transfer of signals by using one frequency for both transmission and reception.
According to the TDD method, as shown in FIG. 1 for example, a single channel (frequency) is divided into a transmission slot T and a reception slot R with respect to time, and such slots T and R are alternately repeated with a guard time Tg existent therebetween In an exemplary case, each of the transmission slots T and the reception slots R is 1 millisecond, and the guard time Tg is several ten microseconds. And a cordless telephone set (slave set) transmits a signal to a base station (master set) during the transmission slot T or receives a signal from the base station during the reception slot R.
In the TDD system which performs such transmission and reception, the transmitting and receiving circuits of the cordless telephone set are so constituted as shown in FIG. 2.
In this diagram, there are included a transmitting circuit 10, a receiving circuit 20, a system controller 31, and a clock generator 32. The system controller 31 serving to control the entire operation of the cordless telephone consists of a microcomputer. The clock generator 32 generates various timing clock signals and control signals The signals obtained from the system controller 31 and the clock generator 32 are supplied to unshown relevant circuits respectively.
In the transmitting circuit 10, a sound signal Sa is supplied to a transmission processor 11 which sequentially executes analog-to-digital (A-D) conversion, time base compression, addition of control data from the system controller 31, and division into two channels, so that digital data DI and DQ of I and Q channels are taken out every transmission slot T. Such data DI and DQ are supplied as modulating signals to an orthogonal modulator 12. Furthermore a carrier signal (oscillation signal) Si having a frequency fi of 80 MHz for example is outputted from an oscillator 13 every transmission slot T and then is supplied also to the modulator 12.
Thus, the orthogonal modulator 12 produces, during each transmission slot T, an intermediate frequency signal Sit (of intermediate frequency fi) modulated by the data DI and DQ.
The intermediate frequency signal Sit is supplied to a mixer 15, and a local oscillation signal Sc of a frequency fc is generated from a synthesizer oscillator 33 provided for selection of a transmission-reception channel. The signal Sc is supplied also to the mixer 15, while the signal Sit is frequency-converted into a transmission signal St of a frequency fs. In such frequency conversion, the following is executed. EQU fs=fi+fc
where fc=2.6 GHz
The frequency fc or channel is set by controlling the synthesizer oscillator 33 by means of the system controller 31.
The transmission signal St is supplied to an antenna 35 via a signal line of a band pass filter 16.fwdarw.a power amplifier 17.fwdarw.a high-frequency switch 34, whereby the signal St is transmitted to the base station or master set.
In this stage, the switch 34 is selectively changed to a transmission slot T or a reception slot R in response to a control signal obtained from the clock generator 32, and the antenna 35 is selectively used for both the transmitting circuit 10 and the receiving circuit 20.
Meanwhile in the receiving circuit 20, the signal Sr (frequency fs) transmitted from the base station or master set during the reception slot R is received at the antenna 35. The signal Sr thus received is supplied to a first mixer 23 via a signal line of the switch 34.fwdarw.the band pass filter 21.fwdarw.the high-frequency amplifier 22, while a local oscillation signal Sc outputted from an oscillator 33 is supplied to the first mixer 23, so that the signal Sr is converted into a first intermediate frequency signal Sir. In this case, the intermediate frequency of the signal Sir is equal to the frequency fi of the signal Sit.
The signal Sir thus produced is supplied via a band pass filter 24 to a second mixer 25, and simultaneously a second local oscillation signal Sj from a second local oscillator 26 is supplied also to the mixer 25, whereby the signal Sir is converted to a second intermediate frequency signal Sjr (intermediate frequency fj=450 kHz), which is then supplied via a band pass filter 27 to a demodulator 28 so as to be demodulated every reception slot R to become digital data DI and DQ. Thereafter such digital data DI and DQ are supplied to a reception processor 29 where a predetermined process for TDD reception is executed to regain the original sound signal Sb. Control data is also outputted from the processor 29 and is supplied to the system controller 31.
In the TDD type cordless telephone using a submicrowave band as mentioned above, there exists a problem with regard to the stability of the frequency. For example, the requisite frequency stability in transmission and reception is 1 ppm or so under the conditions including a line frequency fs of 2 GHz, .pi./4-shift QPSK modulation, a modulation speed of 80 kbps, delay detection for modulation, and a permissible C/N deterioration of 3 dB.
However, it is practically difficult to realize a quartz oscillator where the frequency stability is less than 1 ppm, and a considerable increase will be unavoidable in the production cost even if such oscillator can be realized.
Therefore an AFC function is a requisite for achieving a cordless telephone set which satisfies the above-described requirements in the submicrowave band.
However, in the TDD system where both the transmission signal St and the reception signal Sr have a burst waveform as shown also in FIG. 1, it is impossible to form an effective AFC circuit
Suppose now that, in the cordless telephone set shown in FIG. 2, a carrier signal St is outputted from the oscillator 13 even during each reception slot R. In this state, due to the frequency of the carrier signal Si and the circuit configuration, the signal Si comes to leak out to the band pass filter 27 via the signal line of oscillator 13.fwdarw.modulator 12.fwdarw.mixer 15.fwdarw.mixer 23 band pass filter 24.fwdarw.mixer 25, thereby causing a signal S39 which is denoted by a broken line in FIG. 2. (Therefore, in the telephone set mentioned above, the carrier signal Si is outputted merely during each transmission slot T.)
The signal S39 thus caused is such as ##EQU1## where .alpha. is a precision of the oscillation frequency fi of the oscillator 13; .beta. is a precision of the oscillation frequency (fi-fj) of the oscillator 26; f39 is the frequency of the leakage signal S39 (frequency in the intermediate amplifier 27); and .alpha.=.beta..
Accordingly the frequency error .DELTA.f of the leakage signal S39 is expressed as EQU .DELTA.f=.alpha.fj
If fi=450 kHz and .alpha.=5 ppm for example, ##EQU2##
The value of such error .DELTA.f is sufficiently small to be ignorable in comparison with the transmission-reception frequency fs.